Process for fabricating miniature capacitors



E. J. BRAJER 3,319,312

PROCESS FOR FABRICATING MINIATURE CAPACITORS May 16, 1967 Original FiledNov. 2, 1961 INVENTOR. Jfimjez Edwin BY 9&0 c/Q1%g.

United States Patent 3,319,312 PROCESS FOR FABRICATING MINIATURECAPACITORS Edwin J. Brajer, Arlington Heights, Ill., assignor to ZenithRadio Corporation, Chicago, 11]., a corporation of Delaware Originalapplication Nov. 2, 1961,. Ser. No. 149,577, now Patent No. 3,280,448,dated Oct. 25, 1966. Divided and this application Nov. 12, 1964, Ser.No. 410,575 4 Claims. (Cl. 2925.42)

This application is a division of application Ser. No. 149,577, filedNov. 2, 1961, now Patent No. 3,280,448, for Process for FabricatingMiniature Capacitors, and assigned to the same assignee as the presentapplication.

This invention relates to electrical capacitors and particularly tosmall high capacity units of the type used in transistor radios andsimilar electronic devices where miniaturizing is desirable.

One type of capacitor which is presently in use is constructed of twoceramic sheets or discs which are joined together with a centralelectrode to form two capacitors in parallel. In the past, the separateceramic sheets have been fired to ceramic maturity, inspected andelectroded separately, and then joined. With this technique thecomponents of the ceramic capacitor are handled a great number of times.

Accordingly, it is an object of this invention to provide an improvedprocess for fabricating miniature capacitors.

It is a specific object of the invention to provide a simplified andinexpensive process of fabrication for miniature capacitors.

It is yet another object of this invention to provide a method ofminimizing capacity variation due to temperature changes.

In accordance with the present invention, a process for fabricating aminiature temperature stable capacitor includes providing a two layerdielectric structure, each of the layers having different temperaturecoefficients of capacity and having opposed major surfaces with spacerelements between the interior major surfaces. The layers. and spacerelements are merged into a unitary composite structure having an innercavity and thereafter the structure is provided with electrodes on itsmajor outer surfaces and the walls of its inner cavity.

The features of the present invention which are believed to be novel areset forth with particularityin the appended claims. The invention,together with further objects and advantages thereof, may best beunderstood by reference to the following descriptiontaken in connectionwith the accompanying drawing, in the several figures of which likereference numerals identify like elements, and in which:

FIGURE 1 is a perspective view partially cut away of a device used forpreparing dielectric layers;

FIGURE 1a is a cross-sectional view of the completed dielectric layer;

FIGURE 2 is a perspective view of apparatus for performing a step of theinventive process;

FIGURE 3 is a cross-sectional view of a single composite dielectricstructure;

FIGURE 4 is a perspective view of the structure of FIGURE 3 showing theplacement of an electrode in its interior cavity;

FIGURE 5 is a perspective view of the structure of FIGURE 4 showing theplacement of exterior electrodes; and

FIGURE 6 is a perspective view of a completed miniaturized capacitorincluding electrode leads.

In accordance with the invention, the process includes the provision ofa two-layer dielectric structure, each of the layers having opposedmajor surfaces with spacer "ice elements between the interior majorsurfaces. In a preferred embodiment of the invention, each of thecomponent dielectric layers is prepared separately by a subsidiaryprocess which optimizes the voltage breakdown properties of thedielectric layer by eliminating pinholes.

The specific apparatus to achieve the above subsidiary process is shownin FIGURE 1 and includes a U-shaped metal frame 10 comprising a base 11and sidewalls 12 and 13. These walls have opposed pairs of slots 14 (notshown) and 15 which are adapted to receive T-shaped blades 16 and 17,the arms of the T of each blade meshing with respective slot pairs 14and 15. Each blade has its bottom edge spaced a predetermined distanceabove base 11.

The above apparatus coats a base sheet 18 with two or more successivelayers of undensified ceramic paint. More specifically, a thin sheet ofbase material 18, which may be composed of a polyester film such asMylar plastic, a trademark of Dupont, is threaded under a doctor blade16 and another doctor element 17 and connected to a driving roller 19which is turned at a relatively slow rate to move sheet 18 in adirection indicated by the arrow. The ceramic paint is loaded in twohoppers 20 and 21 which have outlet nozzles 22 and 23 behind doctorelements 16 and 17 respectively. As sheet 18 is pulled underneath thedoctor elements it receives a first coat of ceramic paint 24 asit'passes under blade 16 and a second coat 25 as it passes under doctorelement 17. The doctor elements are set a sufficient distance apart toallow a sufficient time interval for first coat 24 to partially drybefore second coat 25 is applied over it. The height of doctor element17 above base 11 is greater than the space between the bottom of doctorblade 16 and base 11 to allow the second coat of ceramic paint to beplaced over the first coat.

The completed dielectric layer is shown in FIGURE 1a which illustratesthe plastic base sheet 18 with two coats, 24 and 25, of ceramic paint.The above process thus eliminates pinholes through the dielectric layerby constituting it of a plurality of randomly oriented directlysuperposed sub-layers.

Merely by way of illustration and in no sense by Way of limitation, aceramic paint which was found to give exemplary performance in the aboveprocess is composed of the following ingredients and should be mixed asdescribed below:

500 grams of powder: Grams Barium titanate 402 Strontium titanate 45Calcium zirconate 52 Lanthanum oxide 25 Titanium dioxide 7.5 Cob-alttitanate 2.1

200 grams of Varnish I (see below).

5 Grinding balls in quart porcelain jar.

Mix on 20 revolutions per minute rolls for 70 hours or more. Viscosityshould be 118,000 to 122,000 centipoises. Varnish I has the followingformulae and procedures:

67 grams Low Viscosity Polyvinyl butyral resin;

for example, Butvar from Shawinigan Resins Corp.

325 cubic centimeters Toluene.

Put material in jar and mix on ballmill at 40 revolutions per minuteuntil dissolved.

Viscosity should be 21,000 to 23,000 centipoises (BrookfieldViscosimeter).

Add 1 cubic centimeter alkyl phenyl polyethylene glycol ether; forexample Tergitol NP 14 from Union Carbide and Carbon Corp.

Mix 15 minutes on same ballmill.

3 Add 110 cubic centimeters polyalkylene glycol having a viscosity of600 to 700 Saybolt Seconds at 100 F.; for example Ucon oil HB660 fromUnion Carbide and Carbon Corp.

Add 40 cubic centimeters polyalkylene glycol having a viscosity ofapproximately 2000 Saybolt Seconds at 100 F.; for example Ucon oilHB2000 from Union Carbide and Carbon Corp.

Mix until free of lumps.

Viscosity should be 6,000 to 8,000 centipoises.

After the base sheet 18 has been given the two successive layers ofundensified ceramic paint as described above, spacer elements or ribswhich will support another dielectric sheet are applied to the coatedsheet. The application of the ribs to coated base sheet 18 is shown inFIGURE 2. There, an extruder 26 is supplied with a ceramic paint similarto that used for the base sheet coating but with a higher viscosity.Compressed air extrudes the ceramic into a cylindrical shape similar tospaghetti. Three separate ribs 27, 28, and 29 are laid on the coatedsurface of base sheet 18 as shown in FIGURE 2. If desired an alternativeprocess may be used where the spacer elements are concurrently formedwith the upper coat 25 of ceramic paint.

A two-layer dielectric structure having opposed major surfaces withspacer elements between the interior major surfaces is provided bypreparing a second coated base sheet 18', 24', 25 similar to sheet 18,24, 25 and juxtaposing its painted surface on ribs 27, 28, and 29 asshown in FIGURE 3. Before the application of sheet 18' to base sheet 18,ribs 27-29 are coated with a layer 36 of an appropriate type of adhesivesuch as Varnish I to hold the structure together. The abovejuxtaposition of ceramic coated sheets also forms an inner cavity 30which, as shown in FIGURE 3, is divided by center rib 28.

If it is desired to stabilize the variation of capacity withtemperature, the second dielectric sheet 24, 25' may have a differenttemperature coefficient of capacity so that the overall additivecapacity will show a greater stability than that of either sheet byitself. For example, one sheet may be of a composition with a maximum ofcapacity at approximately 35 C. and the other sheet with a maximumcapacity at 50 C. With the illustrative materials mentioned above, thecapacity of each individual ceramic layer may be varied by changing theamount of calcium zirconate in the ceramic paint.

The two-layer structure shown in FIGURE 3 has been trimmed to removeexcess ceramic which extended past ribs 27 and 29 as shown in FIGURE 2and also has had the plastic base sheets removed. Both of these stepsare accomplished after the structure has been dried to give it somerigidity. In addition to trimming the sides of the structure, it is alsocut lengthwise into rectangular components of a predetermined size asshown in FIGURE 4. The size, of course, is determined by the ultimatevalue of capacitance which will be required, capacity being directlyproportional to the area of the dielectric.

The component structure is next merged into a unitary compositestructure by firing it in a batch or a tunneltype furnace. In otherwords, the ceramic material is fired to ceramic maturity to densify it.For the illustrative composition given, a temperature of at least 2500F. is necessary.

After the components have been fired, electrodes are provided on themajor outer surfaces of the structure and the walls of the inner cavity.Most conveniently, the entire inner cavity 30 is coated with silverelectrode paint 31 as shown in FIGURE 4, while both major outsidesurfaces are provided with silver coated electrodes 32 as shown inFIGURE 5.

The two outer electrodes 32 are preferably conductively connected by apair of integral silver strips 33, as shown in FIGURE 5, to enable thecomposite structure to be utilized as two capacitors in parallel. The

structure as shown in FIGURE 5 is now a completed capacitor.

The final step of the process is the provision of leads for thecapacitor. This is shown in FIGURE 6 where a lead 34 is soldered orcemented to electrode 32 and a lead 35 is soldered or cemented in innercavity 30.

Thus, the invention provides a simplified and inexpensive process forfabrication of miniature capacitors.

While a particular embodiment of the invention has been shown anddescribed, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention inits broader aspects, and, therefore, the aim in the appended claims isto cover all such changes and modifications as fall within the truespirit and scope of the invention.

I claim:

1. A process for fabricating a miniature temperature stable capacitorincluding the following steps:

providing a two-layer dielectric structure formed from layers havingdifferent temperature coefficients of capacity and having opposed majorsurfaces, and from spacer elements adapted to intervene between themajor inner surfaces;

merging said layers and spacer elements into a unitary compositedielectric structure having an inner cavity;

and thereafter providing electrodes on the major outer surfaces of saidstructure and the walls of said inner cavity.

2. A process for fabricating a miniature capacitor including thefollowing steps:

coating a first base sheet with a layer of undensified ceramic painthaving spacer elements;

coating a second base sheet with a layer of undensified ceramic paint;juxtaposing said painted surface of said second base sheet on saidspacer elements of said first base sheet to form -a two layer structurehaving an inner cavity;

firing said structure to ceramic maturity to densify such structure;

and thereafter providing electrodes on the outer major surfaces of saidstructure and the walls of said inner cavity.

3. A process for fabricating a miniature capacitor including thefollowing steps:

coating a first base sheet with two randomly oriented contiguous layersof undensified ceramic paint; extruding ceramic ribs;

applying said ribs to the coated surface of said first base sheet;

coating 2. second base sheet with two randomly oriented contiguouslayers of undensified ceramic paint;

juxtaposing said painted surface of said second base sheet on said ribsof said first base sheet to form a two layer structure having an innercavity;

drying said two layer structure and removing said base sheets;

cutting said structure into rectangular components of a predeterminedsize;

firing said components to ceramic maturity to densify such components;

providing electrodes on the outer surfaces of said components and thewalls of said inner cavity; conductively connecting said outerelectrodes;

and providing electrical leads to one of said outer electrodes and tosaid inner electrode.

4. A process for fabricating a miniature capacitor including thefollowing steps:

coating a first base sheet with a layer of undensified ceramic paint;

providing spacer elements for said first base sheet;

coating a second base sheet with a layer of undensified ceramic paint;

juxtaposing said painted surface of said second base sheet on saidspacer elements of said first base sheet to form a two layer structurehaving an inner cavity;

firing said structure to ceramic maturity to densify such structure;

and thereafter providing electrodes on the outer major surfaces of saidstructure and the walls of said inner cavity.

References Cited by the Examiner UNITED STATES PATENTS 2,389,420 11/1945 Deyrup 29-25 .42 X 2,793,333 5/1957 Ehlers 317242 2,899,611 8/1959Bradley 317242 JOHN F. CAMPBELL, Primary Examiner.

WILLIAM I. BROOKS, Examiner.

1. A PROCESS FOR FABRICATING A MINIATURE TEMPERATURE STABLE CAPACITORINCLUDING THE FOLLOWING STEPS: PROVIDING A TWO-LAYER DIELECTRICSTRUCTURE FORMED FROM LAYERS HAVING DIFFERENT TEMPERATURE COEFFICIENTSOF CAPACITY AND HAVING OPPOSED MAJOR SURFACES, AND FROM SPACER ELEMENTSADAPTED TO INTERVENE BETWEEN THE MAJOR INNER SURFACES; MERGING SAIDLAYERS AND SPACER ELEMENTS INTO A UNITARY COMPOSITE DIELECTRIC STRUCTUREHAVING AN INNER CAVITY; AND THEREAFTER PROVIDING ELECTRODES ON THE MAJOROUTER SURFACES OF SAID STRUCTURE AND THE WALLS OF SAID INNER CAVITY.